Before the present applicant, for example in Japanese Patent Laying-Open Publication Serial No.3-135844, has proposed the following type of hydraulically driven vehicle has been proposed. With this hydraulically driven vehicle, when the vehicle movement is accelerated, along with increasing the revolution speed of the engine according to the amount by which the vehicle driving pedal (accelerator pedal) is stepped upon, the flow supply of hydraulic fluid to the hydraulic motor for vehicle movement is also increased. Further, when the driving pedal is released so as to initiate deceleration action, from the point of view of preventing cavitation, along with performing controlled slowing down by which the engine revolution speed is gradually dropped according to the passage of time without directly reducing the engine revolution speed to the idling revolution speed even when the driving pedal is released, also slow return control is performed by which the control valve for vehicle movement is gradually returned to its neutral position.
However, if the above described controlled slowing down is always permitted, various problems arise during specific operations.
(1) The present applicant, in the specification of Japanese Patent Application Serial No. 2-185876, has proposed a hydraulically driven vehicle in which the engine revolution speed can be increased and decreased according to the amount by which a vehicle driving pedal is stepped upon, not only during vehicle movement but also while performing work such as excavation or the like. However if during reduction of vehicle moving speed as described above controlled slowing down is performed, then the controlled slowing down will also undesirably always be in operation during working when the engine revolution speed is adjusted by the use of the vehicle driving pedal, and this will give rise to the problem of bad operational feeling. PA0 (2) Sometimes it can happen that the operator, with his foot poised over the vehicle driving pedal although he has no plans to move the vehicle away from rest, may even for only an instant slightly depress the vehicle driving pedal, so that the engine revolution speed is slightly increased, and the control valve for vehicle movement also slightly opens. At this time, if the above described controlled slowing down control and also slow return control are performed, pressurized hydraulic fluid may be supplied from the control valve to the hydraulic motor.